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explcdt8
01-21-09, 06:09 PM
Last year when on a ride-along, my officer got called to a roll-over MVA. Since there were no injuries, we didn't do a speed measurement, but I asked how one would be done and he said that it would be done by measuring the length of skidmarks, and then driving the patrol car on that stretch of road at various speeds trying to reproduce the skidmarks. There was a specific name for this technique, but I forget what it was called.

Does anyone know the official name for this method, and does your department use it?


Ispbear
01-21-09, 06:22 PM
Well there is a few answers for this. Either he had no idea what he was talking about, he didn't explain it properly, or you didn't understand it properly.

For minimum speed (the minimum speed a vehicle could be traveling to leave those skid marks on that particular roadway) you need the length of the skids and the coefficient of friction for the roadway surface (which can change due to weather, ie. rain, snow, ice, etc.)
To measure the coefficient of friction you can use drag tires or sleds, an electronic measuring device, or test skids.
For determining f from test skids you have the known speed of the cruiser and the known distance of the skid left by the cruiser on the roadway. With those figures you can solve the minimum speed formula for the coefficient of friction. Then use coefficient of friction value to solve for the speed of the vehicle involved in the collision. Also when using test skids you have to perform the test a minimum of 3 times to ensure accuracy.

mcsap
01-21-09, 07:06 PM
We use an accelerometer to get the Coefficient of friction. Drag sleds are so....passe' :)

Speed = the square root of 30 x the distance of the skids x the coefficent of friction.

S = SR 30 ( distance X resistance)

Distance is the measured distance of the actual skids BEFORE impact.

Resistance or coefficient of frction is the " friction " of the road surface ( actually any surface ) taking into account any grade or elevation.

The " average " coefficient of friction of a dry , asphalt road that is level is around......75 to .80.

I was in a pursuit in 1986 and left 155 feet of skids.

It wasn't quite level but it was close.

Speed = the SR of 30 ( d x c)

Speed = SR of 30 ( 155 x .75 )

Speed = SR of 30 x 116.25

Speed = SR of 3487.50

Speed = 59 mph.

I was actually going a little faster but remember , we only take into account skids BEFORE any impact. I slid off of the road and went up on an embankment. I think my speed was close to 60 to 65 mph.

One can also do a Crush anaylsis. We can take a wrecked vehicle and take MULTIPLE measurements showing how far the damage intrudes into the vehicle. After taking these many measurements , we can input them into a software program where identical cars were wrecked and the data was saved. It takes a certain amout of force to push a certain amount of metal/plastic etc inward a certain distance and the program then assigns a speed that was necessary to cause the force which caused the known level of penetration.

We would only go to this degree for a serous or fatal crash.

Accident Investigations can be VERY technical. I am not a reconstructionist ( I am cert as an Advanced Acc Investigator) who even have more training and knowledge in this area.

There are also formulas that take into account different tires being on different road surfaces. The right side tires are on grass while the left side is on asphalt.

And there are also formulas for vehicles that vault. Again , it takes a certain amount of force ( speed) for a vehicle to flip ( vault) and than travel through the air before touching down again. By measuring the distance the vehicle travelled in the air ( and cars do NOT GO AIRBORNE) Along with the weight of the car, a speed can be determined.

We can also do speed calulations if a vehicle spins out and leaves yaw ( not brake) marks from sliding sideways and spinning out.


manahmanah
01-21-09, 07:37 PM
I can barely figure out 8x6.......this is why I am not going to put in for the traffic unit. :cool: Math...is the devil.

mcsap
01-21-09, 08:03 PM
I can barely figure out 8x6.......this is why I am not going to put in for the traffic unit. :cool: Math...is the devil.

8 X 6 = 48 , glad I could help. :)

explcdt8
01-21-09, 09:12 PM
Thanks for the information. I think we were only talking about the test skids made by the cruiser, not the whole procedure.

mobrien316
01-22-09, 01:45 AM
Last year when on a ride-along, my officer got called to a roll-over MVA. Since there were no injuries, we didn't do a speed measurement, but I asked how one would be done and he said that it would be done by measuring the length of skidmarks, and then driving the patrol car on that stretch of road at various speeds trying to reproduce the skidmarks. There was a specific name for this technique, but I forget what it was called.

Does anyone know the official name for this method, and does your department use it?

As already mentioned, skid marks will only give you a minimum speed prior to rollover. What you are really measuring when you measure skid marks is the energy lost by the vehicle as it skidded. Once you determine the coefficient of friction of the roadway, the slope, and the braking efficiency of the vehicle you can quantify the amount of energy lost by the skidding vehicle.

Think of it this way:
You drive a car at high speed into a reinforced brick wall, and the impact is enough to essentially disintegrate the vehicle (the front end crumples four and a half feet, there is induced damage throughout the frame, etc...). Moments before impact you lock up your brakes, causing a skid of ten feet. You might get an energy equivalent speed loss formula something like this:

Speed = Square root of [30 X 10 (the distance) X .75 (the drag factor of the roadway) X 1.00 (the braking efficiency, in this case indicated all four tires locked up)

You would be left with a speed of 15 MPH, which is certainly not enough to disintegrate the car. But that's all the information you can obtain simply from the skid marks.


P.S. - This would have looked a lot better if I could imort my MathType equations into the post, but I don't think I can.

tomwoolworth
01-22-09, 01:54 AM
Remember MATH is the basis for EVERYTHING WE DO!! Embrace MATH.... :)